1. Field of the Invention
The invention generally relates to improved methods of producing bio-fuel and other high-value products from oleaginous biomass such as algae. In particular, the invention provides two-step methods for the production of bio-oil and value-added compounds from algal-biomass using, in step 1, subcritical water extraction at low temperatures to recover co-products such as polysaccharides from a liquid phase; and using, in step 2, high temperature hydrothermal liquefaction of the remaining algae-biomass solids to produce bio-oils.
2. Background of the Invention
Algae as a potential feedstock for developing “drop-in” biofuel have attracted nationwide interest (e.g. development of the National Algae Road Map, the National Biofuel Action Plan, etc.). Unique characteristics of algae which set it apart from other biomass sources are 1) its high biomass yield per unit of light area 2) high oil content 3) oil content can be increased by tuning the culture conditions and 4) production does not require agricultural land, fresh water is not essential and nutrients can be supplied by waste water and CO2.
However the main drawback in algal biofuel production is its cost of production. According to Van Harmelen and Oonk (2006) even with the most favorable assumptions about algal production costs, algal systems dedicated only to production of fuel are not economically feasible. Instead, the economic viability of biofuel depends largely upon the extraction of co-products.
Presently, co-extraction of value added compounds along with the bio-fuel is restricted due to technoeconomic barriers. The major bottleneck is the lack of efficient separation technology. Particularly troublesome is the fact that known methods of lipid extraction and algae biomass conversion (such as organic solvent extraction, hydrothermal liquefaction, and gasification) do not allow the separation of these compounds in their active forms.
Algae biomass is comprised of protein, carbohydrate and fatty acids. Unlike lignocelluloses, the major heating value of the bio-oil produced from the algae feedstock is contributed by the fatty acids. Specifically, if the starting biomass content has a moderate amount of fatty acids (26-30%), then in such a scenario the other two constituents of the biomass do not play a significant role in yield and high heating value of algae-based bio-oil, but rather complicate further processing of the bio-oil.
Hydrothermal treatment and solvent extraction are two processes capable of separating these chemicals in useable form. Solvent extraction has major disadvantages due to the cost and environmental impact of the solvents that are employed. In contrast, hydrothermal (HT) treatment offers a less problematic alternative. HT typically refers to near- and supercritical water systems held under anoxic (reducing) conditions. HT treatments of all types of biomass have resulted in transformation of the bio-molecules to mixtures of gas- and liquid-phase aromatic and aliphatic chemicals (Catallo et al., 2008). HT is attractive for processing algae because, unlike pyrolysis and solvent extraction, it can use wet algae biomass directly without requiring drying the feedstock. Depending on the targeted product states, HT typically involves gasification (HTG), liquefaction (HTL), and/or the use of both HTG and HTL. HTG generally takes place at higher temperatures (400-700° C.) (Peterson et al., 2008) and has the advantage of converting all types of organic molecules to simple gas mixtures such as methane or hydrogen; thus it is not sensitive to lipid content of the algae. However, the breakdown of bio-molecules which occurs as a result of the high temperatures limits the ability of HTG to produce various other high value co-products. HTL, taking place at 200-400° C., produces liquid products, often called bio-oil or bio-crude. HTL has been demonstrated to be effective for producing bio-oil using a range of micro-algae (Dote et al., 1993; Minowa et al., 1995; Yang et al., 2004). The main advantage of HTL is that it can be used to convert other non-lipid organic molecules also to fuel components; thus the total bio-oil yield is greater than the lipid content. For example, Dote et al., (1993) liquefied a strain of micro-algae that contained 50% natural oils at 300° C. with an Na2CO3 catalyst, and were able to produce a yield of 64% (mass basis) oil from this feedstock, showing that not only fat, but also other organics like protein, fiber, and carbohydrate are also converted into oil. However, Selhan Karagoz (2004) employed a low-temperature hydrothermal process to treat biomass (180° C., 250° C. and 280° C.) with reaction times of 15 min and 60 min, and found that during longer reaction times at 250° C. and 280° C., secondary reactions occurred and decreased the yield of oil products, with the majority of compounds in the oil containing less desirable C9-C11 carbons. Thus, a major disadvantage of HTL is that it results in production of a mixture of smaller carbon molecules, together with protein derivatives, resulting in production of bio-oils of lower quality than desired.
There are several studies which showed the channeling of carbohydrate components of biomass to bio-oil by using organic acid; however, and this procedure increases the total bio-oil yield by only about e.g. 3-4%. As mentioned in those reports, organic acids enhance the decarboxylation of the carbohydrate which further repolymerizes into complex structures which also form part of the bio-oil (Ross et al. 2010).
Compared to carbohydrate, protein in the biomass has higher thermo chemical bio-oil conversion efficiency. Ammonia produced via deamination of proteins acts both as a basic catalyst and a reactant, and shifts the sugar degradation mechanism from aqueous pyrolysis (which results predominantly in furan formation) to aldol and related condensation pathway (Nelson et al., 1984), leading to the production of more oily products. However, these chemical phenomena also introduce obnoxious nitrogenous compounds into the bio-oil. Removal of such nitrogenous compounds requires a complex and expensive denitrogenation process. Furthermore, in hydrothermal media under high temperature, carbohydrate/protein produces several toxic chemicals like furfural, hydroxymethyl furfural, nitrogenous aromatic compounds, etc. Due to the presence of such compounds, nutrient recovery and recycling of the aqueous phase becomes difficult. Hydrothermal pretreatment at lower temperature (at which solvolysis/hydrolysis are the dominant reactions) will remove carbohydrate/protein components prior to their conversion into such toxic chemicals. The removal of carbohydrate enhances the physical contact between water and lipid molecules, and increases the extraction efficiency (Libra et al., 2011).
There is thus a pressing need in the art to develop superior methods of producing high quality bio-oil and value added co-products.